Open up about:config and go to layout.css.prefers-color-scheme.content-override and set it to one of the following:
0 => Force a Dark background
1 => Force a Light background
2 => Follow the System theme (default in Firefox 94)
3 => Follow the Browser toolbar theme (default in Firefox 95+)
Lmao at the mild abuse OP is getting from DarkMode fanatics. Though I do get it. FUOP!
“And I took it personally”
AS YOU SHOUUUULD